(a) Field of the Invention
The present invention relates to an objective lens unit favorable for the use of an optical pickup device incorporated in an optical information recording/reproducing apparatus for recording information in optical information recording media such as optical disks and optical cards or for reproducing the information recorded in such information recording media.
(b) Description of the Prior Art
In order to correctly record or reproduce information, an optical information recording/reproducing apparatus needs to be controlled so that a light spot can exactly follow an information track on an information recording medium. For this reason, an objective lens system is movably supported in a focusing direction parallel with its optical axis and a tracking direction transverse thereto so that the position of the lens system is shifted in both the directions by an electromagnetic driving device.
To facilitate such control, it is desired that the objective lens system is compact and light in weight, and recently in particular, this demand has been remarkably increased in accordance with the compaction of the optical information recording/reproducing apparatus. This is because, if the objective lens system which is compact and light in weight is employed, the electromagnetic driving device controlling the lens system can also be used as a small-sized one and as such extreme compaction of an optical pickup device is brought about. As a result, the reduction of the number of lenses constituting the objective lens system is rapidly made and, at last, an objective lens system comprising a single aspherical lens has been realized.
However, problems have been encountered that the single lens is not necessarily lightest in weight in view of the materials and thickness of the lens and is rather heavier in particular consideration of the electromagnetic driving device in addition thereto. This respect will be described in detail below.
FIGS. 1A and 1B show an example of an objective driving device used for conventional optical pickup devices. In these figures, an objective lens system 1 is fixed to a lens holder 2A molded integral with a plastic moving member 2. The moving member 2 possesses a frame 2B rectangular in cross section in such a manner as to surround the lens holder 2A and sets the lens holder 2A upright from the bottom wall of the frame 2B. Further, on the outside of the frame 2B are constructed walls 2F and 2G for forming openings 2D and 2E into which inner yokes described later are inserted. Moreover, supporting portions 2H-2K supporting resilient supporting members 3A-3D are formed respectively on the side walls facing to each other (however, reference numerals 2K and 3D are not shown). On the periphery of the frame 2B are secured tracking coils 4A, 4B and a focusing coil 5 for generating electromagnetic driving power in the tracking and forcusing directions, respectively. The moving member 2 is constructed in this way and connected to a stationary member 6 through the resilient supporting members 3A-3D so as to be displaceable in the focusing and tracking directions.
The stationary member 6 is provided with a base 7A and outer yokes 7B, 7C made from a magnetic material, inner yokes 8A, 8B formed likewise from the magnetic material, permanent magnets 9A, 9B, and a supporting plate 10. The outer yokes 7B, 7C are constructed by being vertically bent from the base 7A to secure the permanent magnets 9A, 9B on the inside thereof, respectively. The supporting plate 10 is secured by a screw 12 on the outside of the outer yoke 7C. On the opposite sides of the supporting plate 10 are formed grooves 10A-10D into which the end portions of the resilient supporting member 3A-3D are fitted.
On the other hand, a circular projecting portion is provided at the center of the base 7A, through which an opening 7Q for an optical path of the objective lens system 1 is bored. Also, the inner yokes 8A, 8B are constructed integral with a disk 11 made from the magnetic material having an opening corresponding to the opening 7Q.
The stationary member 6 is constructed as stated above and is such that the one inner yoke 8A is inserted into the opening 2D of the moving member 2 and the other inner yoke 8B into the opening 2E to configure a first magnetic circuit in which a magnetic flux traverses a vertical guide for the tracking coil 4A and the focusing coil 5 by the base 7A, outer yoke 7B, permanent magnet 9A, and inner yoke 8A and a second magnetic circuit in which the magnetic flux traverses likewise a vertical guide for the tracking coil 4B and the focusing coil 5 by the base 7A, outer yoke 7C, permanent magnet 9B, and inner yoke 8B.
Now, when an electric current flows through the tracking coils 4A, 4B and the focusing coil 5, these coils are shifted in the tracking and focusing directions by a so-called Lorentz force so that the position control of the objective lens system is performed.
In the case where the objective lens system comprising a single lens is attached to such an objective lens driving device, difficulties have arisen that the weight balance of the moving member 2 is lost to cause a rolling phenomenon and, as a result, complete performance of the objective lens system cannot be exerted. FIG. 2 is a view showing schematically a state of mounting of the objective lens system to the lens holder 2A. In order to maintain a space (working distance) between the objective lens system and the information recording medium, not shown, located at the upper portion of the figure, the objective lens system 1 needs to be attached to the upper end of the lens holder 2. Consequently, even though the driving center in tracking drive coincides with the center of gravity c of the moving member 2, the center of gravity compounded of the moving member 2 and the objective lens system 1 fails to coincide with the driving center because the center of gravity gc does not coincide with the driving center, and the moving member 2, which does not move in parallel when a driving force is applied, causes the lens holder 2A to move in such a manner as to shake its head. Some provision for preventing this defect is that a balancer 20 is provided on the lower side of the moving member to correct the unbalance of weight which is attributable to the mounting of the objective lens system 1. This, however, results in increased weight of the moving member 2 and a heavy loss of the effect that the number of lenses constituting the objective lens system has been reduced. Moreover, since a glass material with considerably high refractive index is employed for the single objective lens from the necessity of aberration correction and is relatively heavy for one lens, total weight extremely increases when the balancer is further added, and an excessive load occurs in the driving device. This matter will be explained by the following concrete example.
FIG. 3 is a view for explaining detailedly the position of the center of gravity of the moving member 2 shown in FIG. 2. Now, considering such an objective lens as proposed in the eighth embodiment of Japanese Patent Preliminary Publication No. Sho 61-88213 as the objective lens system 1, when the outer diameter of the lens is 5 mm.phi. and the materials are LASF 08, the weight of the lens is 360 mg and the center of gravity gc is positioned at a distance of 2.08 mm from a first surface of the lens. On the other hand, the height of the lens holder is assumed to be 6 mm in dimension and the center of gravity c of the moving member 2 to be positioned at a distance of 2.75 mm from the lower surface thereof.
In the case where the objective lens system 1 is attached to the top of the lens holder 2A, the weight of the balancer 20 necessary for the coincidence in position between the center of gravity gc of the objective lens system 1 and the center of gravity c of the moving member 2 is 253 mg (=1.58.times.360/2.25) and the total weight of the objective lens system 1 and the balancer 20 is as heavy as 613 mg. Since the acceleration at which the moving memer of the objective lens driving device is driven is proportional to the weight of the moving member, the objective lens system and the balancer having a weight of more than 600 mg in total renders the moving member heavy to reduce the acceleration available or to need an energetic driving system for securing similar acceleration, with the result that many problems will be encountered such as the reduction of performance, raise of cost, and large-sized structure.
As depicted in FIG. 4 for another provision, it is considered that the moving member per se is made thinner to minimize the deviation in position of the center of gravity between the objective lens system and the moving member. This design, however, will create new difficulties such that, since a distance d between two resilient supporting members 3A, 3B turns extremely short, torsional rigidity decreases and the driving force weakens with the reduction of driving sensitivity because the coil attachable to the moving member is inevitably made small.
If the objective lens system is composed of two lenses, the unbalance of weight distribution can be avoided in some extent. Although the objective lens system for optical disks comprising two single lenses is known from Japanese Patent Preliminary Publication No. Sho 59-49512, the objective lens system designed in consideration of the weight distribution of the driving device has never been proposed in the past. That is, the lens system of Sho 59-49512 is composed of two lens components and two lens elements with a view to making aberration correction, is not intended for two-lens-component structure to make the center of gravity of the lens system approach toward that of the holder, and needs the balancer. Further, power distribution of a first lens is large and the necessity of a long focal distance for maintaining a working distance makes it difficult to prepare a compact and lightweight design.
Although Japanese Patent Preliminary Publication No. Sho 63-10118 also provides the lens system of the two-lens-component structure, it is adapted to correct chromatic aberration, using the lenses that .SIGMA.d/f is large and weight is heavy. In addition, since all of astigmatism, spherical aberration and sine condition are forcedly corrected in the lens system, an aspherical surface assumes a complicated configuration, which has an inflection point.
As a result, workability is considerably lowered.
When the focal length of, for example, Embodiment 4 of Sho 63-10118 is proportionally increased to f=4.3, the values of a lens weight mc and a working distance WD are as follows: EQU mc=676 mg, WD=1.35
These values, however, indicate that the lens system is unsuitable for the objective lens for optical memories.
The grounds for f=4.3 are that the best imaging performance has been brought about when a disk thickness is taken as 1.2. Also, speaking of the lens weight, the outer diameter of a first lens component is taken as .phi.5.2 and a second lens component as .phi.4.6, and working glass materials are SFL 03, LaK 18, and SK 5 whose specific gravities are used.